1. Field of the Invention
The present invention relates to a spark plug used for providing ignition of an internal combustion engine.
2. Description of the Related Art
As a spark plug used for providing ignition of an internal combustion engine such as an automotive engine, many spark plugs of the type in which a noble metal chip consisting primarily of Pt, Ir, or the like is welded to a tip end of an electrode have been proposed. This configuration is employed because a discharge portion of the spark plug is arranged so as to protrude into a combustion chamber in order to enhance ignitability of the spark plug, and hence the discharge portion is exposed to a high temperature.
Recently, engine heads tend to have a complicated structure, and a space which is in the vicinity of a valve, and in which a spark plug is to be mounted, is reduced. Therefore, there is an increasing demand for a small spark plug in which the outer diameter of a fitting thread portion of a metal shell is reduced to 12 mm or smaller. When the inner diameter of a metal shell is reduced, the distance between the side face of a center electrode protruding from an insulator, and the inner face of the metal shell is shorter than in a conventional spark plug. A problem thereby arises in that so-called side discharge in which a spark is generated between the center electrode and the metal shell easily occurs. When such a side discharge occurs, spark discharge is not generated in the spark discharge gap, thereby adversely affecting ignitability. In order to prevent side discharge, various techniques have been proposed.
In the spark plug of JP-A-2000-243535, it has been found that, in the configuration in which the discharge portion is made of a refractory noble metal chip, wear of the discharge portion does not largely advance even when the heat dissipation property of the center electrode is somewhat impaired by reducing the diameter of the center electrode. Therefore, the distance between the side face of the center electrode and the inner face of the metal shell is sufficiently ensured by reducing the diameter of the center electrode, whereby the occurrence of side discharge is reduced.
In order to further enhance output and improve fuel consumption, newer internal combustion engines require an increase in temperature and pressure in the combustion chamber, and must allow for ignition of a lean fuel mixture. In the spark plug of JP-A-2000-243535, however, there is a possibility that side discharge cannot be sufficiently eliminated under these conditions.